Interest in separation and recovery of nitrogen from a hydrocarbon gas stream, which contains variable amounts of nitrogen, comes primarily from recovery of nitrogen from gas streams associated with enhanced oil recovery (EOR) projects employing nitrogen for miscible flood of oil reservoirs. In these miscible flooding projects a nitrogen rejection unit (NRU) for producing a nitrogen product pure enough to allow the recovered nitrogen to be reinjected into the oil reservoir is required. In addition the NRU must have capacity for handling the gaseous feed mixture with a minimum of equipment changes while the nitrogen content of the recovered gas changes widely during the comparatively long life of the enhanced recovery project, and also while providing a fuel gas product stream of acceptable heating value, for example 875 btu/scf.
In response to the nitrogen recovery problem associated with enhanced oil recovery projects, several methods of separating nitrogen from methane have been developed. A commonly used method employs an integrated dual distillation column arrangement in which a high pressure column provides a rough nitrogen/methane split and a low pressure column makes the specification product. Generally these prior art methods have been designed for gaseous mixtures having a relatively low concentration of heavy hydro-carbons and/or a relatively unchanging nitrogen concentration in the gaseous mixture being processed, and therefore require equipment changes during the duration of the enhanced oil recovery project to accommodate the changing levels of nitrogen present in the gas to be processed.
Accordingly, it is an object of this invention to provide an improved integrated dual distillation system for removing nitrogen from a gaseous mixture containing nitrogen and hydrocarbon components.
It is another object of this invention to provide an improved process for removing nitrogen from natural gas wherein the nitrogen concentration in the natural gas may vary from the naturally occurring concentration to as high as 75% or more.
It is yet another object of this invention to provide an improved process for treating a miscible flood gas wherein the miscible flood gas contains nitrogen along with a relatively high concentration of heavy hydrocarbons.